1. Introduction
This invention relates to a method and apparatus for bending or otherwise treating glass sheet with directed energy heating It is especially advantageous in the shaping of glass sheet into complex curvo-planar forms, such as for motor vehicle vision units and the like.
2. Background Art
Currently, most mass produced curvo-planar glass sheet products, such as motor vehicle windshields and the like, are formed by a continuous flow gravity bending process. This process employs a large tunnel type furnace or lehr in which the glass is heated to its softening point while being supported only along its periphery by any of various well known types of bending fixtures. In the manufacture of a motor vehicle laminated windshield, for example, the process typically starts with pairs of pre-cut sheets or templets of glass being positioned on a bending fixture, such as a skeleton-type bending fixture comprising hinged "wings" in accordance with designs well known to the skilled of the art. The bending fixture assists in bending the glass sheet, often most sharply on each side proximate the short or "pillar" edges of the windshield. The actual bending of the glass is due to the combination of the force of gravity pulling down the softened glass and the localized mechanical leverage exerted by the hinged wings of the fixture. Each different windshield size and shape usually requires its own uniquely designed bending fixture.
The typical glass sheet heating and bending process can be divided into three distinct phases, each with its own requirements and optimal conditions. In most industrial processes of this type, a series of glass sheets is moved through the three phrases of the process without interruption and at a constant speed. In the first phase, the preheat phase, cold glass sheets are heated from ambient or room temperature to an elevated temperature near the softening point of the glass, typically about 560.degree. Celsius. Often the preheat phase requires between two-thirds and three-quarters of the total heating time. During this phase it is desirable to heat the entire glass volume as uniformly as possible to avoid the creation of large temperature gradients and potentially damaging thermal stresses within the glass.
During the second, most critical phase, the bending phase, the actual bending (and often associated stretching) of the glass sheet takes place. Typically, the required temperature increase over the preheat temperature is only about 50.degree. Celsius. It is well known to those skilled in the art that very close control of the heating rates and duration or residence time of the glass within this phase is required for successful, reproducible glass bending. Thus, for example, for the thin glass sheets (e.g. 1.8 to 2.3 mm) typically used in motor vehicle laminated windshields, the residence time in the bending phase is often a mere 60 to 90 seconds.
In the third phase, the so called annealing phase, the heat resident in the glass sheet is removed in a controlled fashion. The objective generally is to remove the heat uniformly from all areas of the glass to prevent harmful permanent stresses building up in the finished glass product. Ideally, the entire glass volume should "refreeze" at exactly the same point in time. Thus, while refreezing could be achieved within less than 30 seconds of exiting the bending phase, as a practical matter, it is impossible to refreeze the entire glass volume simultaneously and for this reason the cooling process is slowed.
Generally, the heat energy required to soften the glass is generated in the lehr either by electrically powered metal resistance elements, closed gas fired radiant tubes, or open-flame radiant gas burners, all of which are well known to the skilled of the art. These energy sources typically are permanently fixed in various arrays approximately 0.5 to 0.7 meters above the horizontal plane in which the glass sheet moves through the lehr. Occasionally, so called lower side wall heaters or radiant tubes located below the glass movement plane also are used. Due to the wide variety of sizes, bend locations and degrees of curvature required for curvo-planar glass sheet products such as motor vehicle windshields and the like, it is well recognized that the thermal energy required for the heating and bending process is preferably distributed nonuniformly and uniquely for each different product design. This often is accomplished by the use of a number of auxiliary heaters within the lehr which can be moved laterally and/or vertically with respect to the lehr centerline. This relatively simple and economical process, with various modifications, is now in worldwide use in the manufacture of motor vehicle windshields and other vision units. Glass templets, usually in matched pairs, are loaded onto a bending fixture as described above and sent through the lehr oriented such that the vertical centerline of the windshield is parallel to the direction of travel of the glass through the lehr. Continuous flow gravity bending of conventionally shaped windshields is achieved in this way at rates often as high as four to five bent pairs per minute.
The conventional gravity bending technology described above has limitations which make it impractical or inadequate for forming glass sheet products of certain configurations. Complex curvo-planar vision units may have, for example, a deep, abrupt or asymmetric curvature. Such configurations may not be possible using only the force of gravity in the conventional glass bending methods and apparatus described above. Motor vehicle windshield glass, for example, has become progressively thinner and simultaneously larger in vertical and lateral dimensions. This has resulted in difficulties in controlling the final glass form or profile, most notably in areas away from the glass edges supported by the bending fixture. This is believed due at least in large part to an intrinsic feature of current bending process, that is, that the periphery of the glass sheet heats faster and to a higher temperature during the bending process than does the interior of the glass sheet. This is caused by a combination of conductive heat transfer from the metal bending fixture on which the glass sheet is supported, preferential convective heating of the glass edges by the circulation of ambient gases in the lehr, and the more efficient radiant heat absorption by the dense black paint often used on the peripheral areas of windshields. Also, there is the inherently higher radiant heat transfer to any open glass edges and corners. As a result, the sag, that is the surface deformation along and parallel to the vertical windshield centerline, generally must be held to a very low value (i.e., it must be nearly flat) or the deformation will have the general shape of a wide, flat bottomed "U". In extreme cases this can even result in a noticeable reversal of curvature. Such flat spots or reverse curvature areas on an otherwise curved surface are undesirable not only esthetically, but also functionally. In extreme cases, for example, a reverse curvature may result in a windshield wiper bridging over an area and failing to properly clean the glass.
The inherent overheating of peripheral areas of glass sheets in conventional processing in some cases also creates, upon cooling, visually unacceptable wrinkles or "pie crust" configuration along the windshield edges. It also causes the peripheral areas to be softer, resulting in deep tool imprints or "mold marks" in the surface of the glass where contacted by glass bending fixtures or handling tools. Also, the periphery heating problem makes it more difficult to achieve consistent control of final product configuration, especially in the area of the acutely angled corners or "horns" frequently found at the upper corners (i.e., at the pillar roof intersections) of motor vehicle windshields of advanced design.
To date, various attempts to overcome the problems caused by improper heat distribution in the production of curvo-planar glass products have included the placement of heat sinks or "heat robbers" into the glass bending fixture below the glass sheet to function by absorption, shielding or reflection of thermal energy. Another approach has been to shield selected areas of glass from above. Neither of these approaches has proven to be completely satisfactory, since both interfere with the production process and the ease of its operation. Both reduce the efficiency of heat transfer and both may present serious obstacles to automation of fixture loading and unloading. Furthermore, the bending of a glass sheet is fundamentally dependent on the lowering of the viscosity of the glass material by the elevation of temperature and furnace residence time. While slight or modest bends can be achieved at either lower temperature or in shorter time, deep and/or complex bends require either a higher material temperature or a longer bending time. Experience has demonstrated, however, that it is difficult to control product configuration and quality, particularly the avoidance of optical defects, in a conventional process using high temperatures and short bending times.
A particular problem in the production of windshields and other vision units of advanced design involves the occurrence therein of nonparallel bending axes. Where a glass sheet progresses continuously through a conventional bending lehr, in addition to being sag formed by gravity into a generally convex shape, each of its two lateral side areas also may be bent about a bending axis to give the finished windshield product a "wrap-around" effect. Such bending axes, however, typically are generated, as described above, by passing the glass sheet under a source of concentrated heat energy, one on each side in line with the intended location of the bend. It will be apparent as a matter of simple geometry, therefore, that such bend axes will be parallel to each other and to the line of travel of the glass sheet through the bending lehr. Advanced windshield designs, however, call for windshields more narrow at the top. Consequently, their bending axes, rather than being parallel to each other and to the centerline of the lehr, must converge toward each other at the top of the windshield. Using the conventional bending lehr process and apparatus described above, the aforesaid source of concentrated heat energy provided for each intended bend axis might be caused to move progressively toward the lehr centerline in synchrony with the movement of the glass past it. As a matter of process engineering and economics, however, this often is not practical and may not even be feasible. Accordingly, it is a particular object of certain preferred embodiments of this invention to provide a method and apparatus suitable for the production of curvo-planar glass sheet products having converging bend axes.
More generally, it is an object of the present invention to provide a method and apparatus for the treatment of glass sheet in the production of curvo-planar glass sheet products. It is an object of certain preferred embodiments of the invention to provide a method and apparatus capable of producing curvo-planar glass products having good optical quality, even in the case of relatively large products of complex configuration.
The above-stated and other objects of the invention will be better understood from the following disclosure and discussion of the invention, taken in conjunction with the appended drawings.